The present invention relates generally to an ultra-sonic distance sensor for measuring the distance to an object capable of measuring the elapsed time between transmission of an ultra-sonic wave pulse and receipt of reflected ultra-sonic waves. More specifically, the invention relates to an ultra-sonic distance sensor system which satisfactorily and successfully avoids the adverse influence of directly transmitted ultra-sonic waves.
Published Japanese Patent Application (Tokkai) Showa No. 56-153268, published on Nov. 27, 1981 discloses an ultra-sonic sensor which measures distance by means of ultra-sonic waves. A similar system has been disclosed in Published Japanese Patent Application (Tokkai) Showa No. 56-153267, published on Nov. 27, 1981, in which an estimated maximum distance can be used to determined the discharge of transmission timing of the ultra-sonic sensor wavepulses or packets.
Published Japanese Patent Application (Tokkai) Showa No. 57-182544 and Published Japanese Utility Model Application (Jikkai) Showa No. 55-141085 disclose an ultra-sonic distance sensor used to detect road clearance in front of a vehicle. Application of an ultra-sonic sensor to a height-control system for automotive vehicles has also been disclosed in the European Patent First Publication No. 0 091 017, published on Oct. 12, 1983, which corresponds to the co-pending U.S. patent application Ser. No. 476,519 now U.S. Pat. No. 4,518,169.
In each of these ultra-sonic distance sensors, a ultra-sonic wave transmitter and a receiver are arranged immediately adjoining one another, e.g. in a side-by-side relationship. This allows some of the transmitted ultra-sonic waves to be directly transmitted to the receiver, which interferes with accurate measurement of the distance. This adverse influence on measurement due to directly transmitted ultra-sonic waves must to be overcome for accurate measurement of the distance to the object.
Published Japanese Patent Application (Tokko) Showa No. 58-28554 proposes an ultra-sonic distance sensor with a feature designed to eliminate the effect of the directly transmitted ultra-sonic waves. In the disclosed system, a reference signal is produced in synchronism with transmission of the ultra-sonic wave toward the object. The reference signal varies exponentially with time. The receiver signal level is compared with the reference signal value to detect the reflected ultra-sonic wave while avoiding components due to ultra-sonic waves transmitted directly from the transmitter to the receiver.
Although the aforementioned system reliably eliminates the effect of directly transmitted ultra-sonic waves, a drawback is encountered in that the receiver signal level may drop below the reference signal level due to low target reflectivity or propagation losses. This is reflected in an apparent infinite distance measurement.